Sensor and Method for the Production Thereof

ABSTRACT

The invention concerns a sensor with silicon-containing components from whose sensitive detection element electrical signals relevant to a present analyte can be read out by means of a silicon semiconductor system. The invention is characterized in that the silicon-containing components are covered with a layer made of hydrophobic material in order to prevent unwanted signals caused by moisture.

PRIORITY INFORMATION

This patent application claims priority from International patentapplication PCT/EP2005/050418 filed Feb. 1, 2005, German PatentApplication No. 10 2004 005 927.6 filed Feb. 6, 2004, and German patentapplication 10 2004 035 551.7 filed Jul. 22, 2004, which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates in general to sensors and in particular to asensor, for example a gas sensor, acceleration sensor, or pressuresensor, with components containing silicon, where electrical signals canbe read when analytes are present or in case of mechanical deformation.Moisture in the air forms a thin film of water on the surface ofmaterial containing silicon, which leads to increased surfaceconductivity. Leakage currents from this increased conductivityrepresent a problem with regard to stability and signal quality for manysensors that are in contact with air.

To prevent the effects of moisture on sensor systems, the sensors areencapsulated if possible. If contact with the surrounding air isnecessary for proper operation of the sensor, for example gas sensors,passive water-repellent membranes may be used. Also, heating totemperatures well above 100° C. solves the problem, but this isassociated with considerable expenditure of energy.

There is a need for a sensor with a semiconductor body whose moisturesensitivity and leakage current are substantially reduced.

SUMMARY OF THE INVENTION

Silanization familiar from glass coating can also be applied tosemiconductor technology. In this case, a monolayer of the hydrophobicmolecular chains that suppress the adsorption of water molecules isformed on a surface containing silicon. All hydrophobic molecular chainsthat participate in a stable bond with the surface are suitable for thisapplication. Thus, no continuous water film that favors the unwantedsurface conductivity can form, up to high atmospheric humiditylevels—almost 100%.

Structural elements containing silicon can operate in ambient air aftersilanization without heating or encapsulation, and without the problemof interference from surface currents induced by moisture.

In general terms, the semiconductor body used as the base in thissilicon technology is silanized. Either pure silicon or siliconcompounds present superficially can be treated.

The fields of use of such semiconductor sensors based on silicon andinsensitive to moisture, for example, are gas sensors, pressure sensors,or in general any sensors that come into contact with essentiallyatmospheric moisture when in operation. Hence analytes such as targetgases are detected by gas sensors, and mechanical shape or deformationchanges are detected by pressure sensors or acceleration sensors.

These and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of preferred embodiments thereof, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph that illustrates a comparison of a silanized hydrogensensor and a sensor with no hydrophobic covering layer;

FIG. 2 is a graph that illustrates various humidity levels andadditional gases; and

FIG. 3 is a cross-sectional illustration of a gas sensor in the form ofa floating gate FET.

DETAILED DESCRIPTION OF THE INVENTION

The functional principle of silanization on silicon nitride and oxidizedpolycrystalline silicon was tested specifically on a gas sensor, inparticular a floating gate field effect transistor (FGFET). Otherembodiments of FETs can also be used, for example suspended gate FETs.FIG. 3 illustrates schematically the structure of the FGFETs used. Thepotential change occurring on a sensitive layer from gas impingement isfed to the MOSFET by the voltage divider extending between a floatinggate and a capacitive well (electrode), and leads to a current changebetween a drain and a source. A floating electrode (gate) is coveredwith a nitride or oxide layer to protect it against interfering leakagecurrent. Nevertheless, potentials can still be coupled in capacitivelythrough a conducting moisture film on this passivation. To prevent this,an equipotential surface, for example a guard ring, is placed on thesurface around the sensitive gate. At higher atmospheric humidy levels(>50%), increased surface currents nevertheless occur, which lead tosevere signal drift. To prevent this, it is necessary to prevent theformation of a moisture film. Hydrophobic molecular chains are thenapplied to the existing passivation by silanization before a hybrid gateis mounted. Since the adhesive bond of the gate then no longer adheresto this layer, additional aluminum-adhesive pads are necessary on thechip, since the silanization does not adhere there. Because of thisprocess, the unheated gas sensors thus produced are almost completelystable even at high humidy levels. Subsequent measurement shows acomparison between a silanized hydrogen sensor and an untreated sensorat various humidy levels (see FIG. 1).

The sharp drift and “distortion” of the hydrogen signals is effectivelysuppressed by silanization. The remaining small moisture steps in thesilanized signal are caused by the dipole signal of water on thesensitive platinum layer and no longer interfere.

To gain precise information on surface conductivity, the above FGFET wasput together with surfaces with no hybrid gate, both silanized andunsilanized. To measure the very small currents qualitatively, use wasmade of the sensitivity of the floating gate. The guard ring wascontrolled in both chips with a square-wave generator and themoisture-dependent coupling to the transistors was measured. A very lowfrequency was chosen (0.1 Hz) to preclude frequency-dependent effects inthe RC circuits. The higher the surface conductivity, the larger thecoupling of the square-wave generator into the transistor. FIG. 2illustrates a comparison of these measurements with various humidylevels and additional gases. The current in the transistors is keptconstant by feedback electronics. The resulting signals originate fromthe feedback control circuit and thus show the potential applied to thefloating gate.

It can be seen that all moisture effects have disappeared aftersilanization. The remaining coupling is only capacitive. The reaction ofthe nitride to NO₂ has disappeared in the silanized version. Increasedsensitivity to NH₃ exists instead. This is to be expected with thetrichlorosilane used as the starting material for silanization,especially n-octadecyltrichlorosilane, since alkalis like ammonia attackthe bonds to the nitride passivation. On the other hand, the layer isespecially stable to acids (for example, NO₂). The samples with oxidizedpolysilicon show the same behavior.

Although the present invention has been illustrated and described withrespect to several preferred embodiments thereof, various changes,omissions and additions to the form and detail thereof, may be madetherein, without departing from the spirit and scope of the invention.

IN THE DRAWINGS

The attached sheet of drawings includes changes to FIGS. 1-3. Thesesheets replace the original sheets that included FIGS. 1-3.

1. A sensor comprising a plurality of components containing silicon andhaving a sensitive detection element, where electrical signals are readby a silicon semiconductor system, where the components containingsilicon are coated with a layer of hydrophobic material.
 2. The sensorof claim 1, where hydrophobic layer comprises molecular chains that forma stable bond to silicon.
 3. The sensor of claim 2, where.
 4. The sensorof claims 1, where the components containing silicon comprise silicon,silicon nitride, or oxidized silicon.
 5. The sensor of claim 1, wherethe silicon semiconductor system comprises a field effect transistor. 6.The sensor of claim 1, where the sensor comprises a sensor from thegroup including a gas sensor, a pressure sensor, and an accelerationsensor.
 7. A method for producing a gas sensor with a gas-sensitivelayer integrated in a field effect transistor (FET) with componentscontaining silicon, on which layer electrical signals corresponding to atarget gas that is present are read by the FET, the method comprisingthe steps of: coating a plurality of components containing silicon witha hydrophobic layer by silanization; and mounting additional componentsbelonging to the FET.
 8. The method of claim 6, where silane is used forthe silanization.
 9. The method of claim 7, where trichlorosilane isused for the silanization.
 10. The method of claim 8, wheren-octadecyltrichlorosilane (C₁₈H₃₇Cl₃Si) is used for the silanization.11. A sensor comprising at least one component containing silicon andhaving a sensitive detection element, where the at least one componentcontaining silicon includes a coating layer of hydrophobic material. 12.The sensor of claim 11, where the hydrophobic coating layer comprisesmolecular chains that form a stable bond to silicon.
 13. The sensor ofclaim 12, where the molecular chains form a monolayer.
 14. The sensor ofclaim 11, where the sensor comprises a gas sensor.
 15. The sensor ofclaim 11, where the sensor comprises a pressure sensor.
 16. The sensorof claim 11, where the sensor comprises an acceleration sensor.
 17. Thesensor of claim 11, where the hydrophobic coating layer is applied bysilanization.
 18. The sensor of claim 17, where a silane is used for thesilanization.
 19. The sensor of claim 17, where a trichlorosilane isused for the silanization.
 20. The sensor of claim 17, where ann-octadecyltrichlorosilane (Cl₈H₃₇Cl₃Si) is used for the silanization.